Composites

ABSTRACT

A flexible composite comprising a high tensile strength fibrous component dispersed within a flexible or resilient polymeric matrix, the matrix and fibrous component being essentially unbonded to each other so that the composite retains essentially the flexibility of the polymeric matrix.

This is a continuation of application Ser. No. 08/236,258, filed May 2,1994, which is a continuation of Ser. No. 07/985,229, filed Dec. 2,1992, now abandoned, which is a continuation of Ser. No. 07/541,971,filed Jun. 22, 1990, also now abandoned.

FIELD OF THE INVENTION

The present invention is concerned with a flexible or resilientcomposite comprising a polymeric matrix having a fibrous componentdispersed therein for reinforcement. The composite is characterized byits high puncture resistance and other useful properties.

BACKGROUND OF THE INVENTION

A great deal of research effort has been, and is currently being,expended towards developing composites of resins and fibrous materialswhich provide needed properties. Typically such composites may includeany one or more conventional resins or other matrix material such asepoxy or polyester resins, reinforced with various types of fibersincluding, for example, glass or metal fibers or the like.

A useful discussion regarding composites appears in an article by Chouet al entitled "Composites" appearing in Scientific American, October,1986, Volume 255, No. 4, pages 193-202. The article describes a varietyof different types of composites comprising fibrous materials dispersedin various matrix materials. The article notes that, in the case of abrittle, ceramic matrix material, a crack in the matrix may cause thereinforcing fiber to fail as well unless the bond between the matrix andfiber is quite weak. Normally, however, steps are taken to provide formaximum bonding between the matrix and fibrous component. This may beaccomplished by appropriate selection of the matrix and fibers and/or bypretreatment of the fibers to provide physical or chemical bonding tothe matrix.

As noted, a variety of fibrous components in various forms, e.g. metal,glass, polyester, etc. in the form of woven, non-woven or knittedfabrics, or as staple fibers or filament bundles, have been proposed forcomposite use. More recently such materials as aramid and extended chainpolyethylene fibers (e.g. "Spectra" fibers) have been proposed for usein composites. However, as far as can be ascertained, all suchpreviously disclosed uses have required adhesion between the fibrouscomponent and the matrix to provide useful flexible or resilientcomposites.

SUMMARY OF THE INVENTION

The present invention is based on the finding that a highly usefulresilient or flexible composite can be obtained by combining a resilientresin component and a fibrous component such that the resin encases orenvelops the fibrous component with essentially no adhesion or bondbetween the two components. This is substantively different from priorcomposites where, as noted, bonding between the resin and fibrouscomponents has been considered desirable, if not essential. In thepresent case, the resin and fibrous components are so chosen that anysignificant amount of bonding does not occur. As a consequence, theresin, which is itself resilient, can retain its resiliency whileperforming the matrix function. At the same time, the fibrous componentadds strength and other desirable properties, particularlypuncture-resistance, to the composite.

Particularly effective results are obtained by forming the resin matrixin situ about the fibrous component which may be in the form of staplefibers, continuous filament, non-woven, woven or knitted fabric.

In a preferred embodiment, the invention contemplates the use ofultra-high molecular weight, high tensile strength, high modulusextended chain polyethylene fibers as the fibrous component and flexiblepolyurethane formed in situ by positioning the urethane-formingcomponents about the fibers and allowing the desired urethane-formingreaction to occur. Such fibers and resin matrix do not bond together,the non-bonding effect being aided by the highly lubricious nature ofthe polyethylene fibers. Polyester fibers may also be usefully employedwith the flexible polyurethane matrix or the like as long as anysignificant chemical bonding between the matrix and fibrous component isavoided. According to the invention, the composite is essentially asresilient as the polyurethane itself until the composite is bent to thepoint where the fibers in the matrix are snubbed, i.e. the matrixcontracts around individual fibers to affect a braking action on theslippage between the matrix and fibers. Up to this point, the compositemay be bent without causing tension on the encapsulated fibers which, ina sense, float within the resin matrix. However, when the bending of thecomposite is such that fiber snubbing or braking occurs, the fibersincrease their reinforcing effect by coacting with fibers in proximitythereto so as to spread the load placed on the composite. Then, when thebending force is released, the energy stored up in the snubbed fibersfacilitates the return of the composite to its prior dimensions. Thecomposite thus, in essence, retains desired flexibility or resiliency ofthe resin component while being reinforced by, and otherwise benefitingfrom, the fibers.

It is to be noted that the manner in which the present compositefunctions on bending and release would not be possible if the fibers andmatrix were physically or chemically bonded together. Thus, significantor intentional adhesion between the fibers and matrix restrictsflexibility and the thus encased fibers might well break before sharingthe bending load with other adjacent fibers. In the present case, thefibers do not change position before, during or after deformation withrespect to the matrix. The fibers instead float unadhered within thematrix until the composite is bent to the point where the fibers arestubbed or squeezed in their position by the bent matrix, the energystored in the thus stubbed fibers helping to spring the composite backto its original form when the bending force is released.

DETAILED DESCRIPTION OF THE INVENTION

A wide variety of resilient polymeric materials may be used to providethe matrix for the present composite. Preferably, however, as notedabove, the matrix comprises a flexible or resilient polyurethane whichis formed in situ by application of the polyurethane-forming reactantsabout the fibrous component followed by reaction and curing. Typically,the polyurethane-forming reactants comprise (A) an aliphatic isocyanate,e.g. an isocyanate prepolymer such as isophrone diisocyanate, ordiphenylmethane diisocyanate and (B) an aliphatic hydroxy component suchas a polyester polyol or a mixture thereof with polypropylene glycol.Any conventional polyurethane-forming components may be used for thispurpose provided the polyurethane reaction occurs at a temperature belowthe melting point of the fibrous component. Preferably, the polyurethaneis formed by separately preheating the reactants (A) and (B) to atemperature of, for example, 30-60° C. and applying these reactantsabout the fibrous component, the latter being positioned in a mold orotherwise supported at ambient temperature (18-32° C.). The resulting insitu reaction is an exothermic one which should be controlled, ifnecessary, to keep the temperature well below the melting point of thefibers involved. Usually, for polyethylene fibers, the temperature willbe kept below about 70° C. while higher temperatures, e.g. up to about120° C. may be observed with low shrinkage polyester fibers.

Polyurethane matrix materials, however, are preferred because they tendto have good abrasion resistance and, in the case of aliphate urethanes,good UV resistance; and in the case of polyethers, good hydrolyticstability.

While polyurethane comprises the preferred matrix, it will be recognizedthat other resins which are resilient may be used. This includes, forexample, vinyl resins, ethylene propylene polymers, epoxies and thelike. A variety of fibrous components or mixtures thereof may be usedfor present purposes. However, as noted, it is preferred that thiscomponent comprise either polyester fibers or ultra high molecularweight, high tensile strength polyolefin fibers. Of particularpreference are extended chain polyethylene fibers, e.g. fibers availableas Spectra 900 and Spectra 1000, which have been found to be especiallyeffective. Such polyethylene fibers have exceptionally high tensilestrength and, because of this, a fabric can be made from this fiber thatis more open than fabrics made from lesser-strength fibers at a givenstrength level. For example, a fabric woven of 1,200-denier polyesteryarn in a 32 by 32 construction (that is, 32 warp and 32 filling yarnsper inch) has virtually no "windows", i.e. there are essentially noopenings therein such as there are in woven window screen fabric. Such aclosed polyester fabric is less strong than an open fabric woven of1,200 denier yarn of Spectra 900 or 1000 fiber in a 10 by 10construction (that is, 10 warp and 10 filling yarns per inch). Thelatter fabric is so open that it has a substantial "window" between eachwarp and filling yarn. This ability to make a strong fabric with"windows", coupled with the lubricious or slippery nature of thepolyethylene fiber, makes this fiber especially useful for presentpurposes.

It is also noted that "Spectra" fiber transmits load faster than thenext most high performance fiber, aramid; nearly twice as fast.Accordingly, when one "Spectra" fiber in fabric form is subjected to aforce, that fiber quickly marshalls its companion fibers, sharing theirassigned reinforcement task at the rate of 12,300 meters per second.Consequently, a runner, for example, stepping on a nail with a shoemidsole made according to the invention brings a myriad of super-strongfibers, automatically and virtually immediately, into protectivebehavior.

The "Spectra" fiber is lighter, i.e. it has a lower specific gravity(0.97) than polyester (1.38) or aramid (1.44) or glass (2.6) or steel(7.0) and still outperforms the other fibers. This is consequentialbecause it enables the present composites to exhibit maximumstrength:weight properties.

It is noted that "Spectra" fiber tends to shrink at the boiling point ofwater and, at 250° F., the fiber starts to weaken slowly but noticeably.As a consequence, the resin selected for use with these fibers should beformed at temperatures well below the boiling point of water,preferably, for example, at essentially hand-washing temperatures (belowabout 70° C.).

"Spectra" fibers or like extended chain polyolefin fibers do not bond tomost resins. Steps have been taken in the past to improve the adhesionproperties of these fibers, e.g. by corona or plasma treatment or byspecial adhesives, in order to form composites because it has beenthought that such adhesion was essential. However, for present purposes,it is important that the fibers not adhere to the resin matrix in orderto obtain a resilient composite. In this regard, some incidentalphysical adhesion may occur between the fibers and the matrix due, forexample, to irregularities in the fiber surface. However, suchincidental adhesion is not the sort which the invention intends to beavoided. The key thing is to avoid chemical adhesion between the matrixand fibrous components.

The present composite may be prepared in a variety of ways. For example,the resin or resin-forming components may be cast or nipped around thefibers or they may be sprayed among the fibrous component and allowed toset or, in the case where the resin is formed in situ, the resin-formingcomponents are permitted to react and cure. Advantageously, two liquidreactant parts of the matrix resin may be preheated and mixed preciselyunder high pressure at the head of an airless spray gun. The thusatomized resin mixture is then deposited on and among the prepositionedfibers, these being placed in a suitable jig or mold or the like.Alternatively, the reactants may be co-sprayed with the fibers and anyother optional components onto a suitable substrate or mold surfacewhere the resin components react to form the matrix with the fibersdispersed therein. It will be appreciated that the substrate may be afabric, paper, film, sheet, foil, metal or the like. In whatever methodis used, the resin should envelop the reinforcing fibers or fibrousbundle without bonding thereto, it being noted that where a bundle offibers is used, the resin may encapsulate the bundle while individualfibers thereof are not all encapsulated.

The fibrous component may be in any convenient form, e.g. as individualfibers, filaments, fiber bundles or non-woven, woven or knitted fabric.The term "fibers" is used for convenience herein although it is intendedthat the term embrace both staple fiber as well as continuous filament,cut to desired length, bundles thereof or fabrics based thereon.

The fibrous component may be used in random or oriented fashiondepending, for example, on the properties desired in the composite. Inthe case of random configuration, staple fibers may be used. Such fiberstend to curl or bend to varying degrees when dispersed within the resinmatrix. As an alternative, continuous filaments may be used in a randomor scrambled fashion.

Preferably the fibers are used in fabric or bundle form and highlyuseful results are obtained when the fibers are straight and parallel ina given plane without crimp or meander. Multiple axes, each in a givenplane, may also be used when the circumstances warrant. When fiberbundles are used, the bundles preferably include 10-1000 filaments withthe bundles arranged in parallel in a common plane. Parallel layers offilaments may also be used with the filaments in each layer oriented atdifferent angles to the filaments in adjacent layers. Preferablystraight or uncrimped fibers are used because any force applied to suchfibers instantly loads the fibers in tension or compression whereascrimped fibers need to be first straightened out by the applied force.

Orientation of fiber layers is used primarily to build strength in thedesired direction(s). Advantageously, the fibers in each layer are inparallel, or essentially so, as this permits the packing of more fibersinto a given volume. It is also preferred that the fibers be positionedso that their largest dimension (length) is parallel to the force it isintended to resist. The matrix serves to keep the oriented fibers inalignment, both individually and in bundle form, even through cycles ofloading the composite in tension and compression and even though all ofthe fibers are not completely enveloped by the resin.

The invention is illustrated, but not limited, by the followingexamples:

EXAMPLE 1

A resin-fiber composite was prepared as follows using a flexibletwo-reactant polyurethane formed in situ as the resin matrix and fabriccomposed of "Spectra" 1000 extended chain polyethylene fibers.

The resin was applied by spraying the two-reactant mix about the flatfabric using a fiber:resin ratio (by volume) of 30:70. The fabric was aneedle-punched, non-woven fabric (7.7 ounce per square yard) of randomlyoriented 2 inch "Spectra" 1000 fibers.

The polyurethane used in this example comprised two reactive componentsas follows:

    ______________________________________                                        Material             Parts by Weight                                          ______________________________________                                        (A)   polypropylene glycol                                                                             50                                                         polyester polyol   50                                                         phenyl mercuric carboxylate                                                                      0.5                                                  (B)   isophorone diisocyanate                                                       (aliphatic isocyanate prepolymer -                                            B.P. greater than 316° F.)                                       ______________________________________                                    

50 Parts of (A) and 50 parts of (B), by volume, were used to form thepolyurethane matrix. These reactants were each separately preheated to50° C. before combining and applying to the fabric. The fabric was atambient temperature but because of the exothermic reaction involved, thetemperature rose to about 60° C. The resulting polyurethane began to gelin about 20-25 seconds and the thus forming composite was subjected topressure to reduce the thickness to below 1 millimeter. A platen presswas used for this purpose although a vacuum device could also be used.The gelling time could also be reduced by increasing the temperature ofthe environment about the composite.

The polyurethane obtained after the reaction was completed had a Shore Ahardness of about 80 and the composite was characterized by itsresilience and puncture resistance.

The above polyurethane can be replaced by a more rigid polyurethane,e.g. one having a Shore D hardness of about 60, to also provide a highlyuseful composite according to the invention.

EXAMPLE 2

A composite was prepared by spraying a two-componentpolyurethane-forming mix onto fabric using a fiber:resin ratio (byvolume) of about 50:50 so as to fully encase the fabric, followed bysurface coating with unreinforced resin.

The fabric used was a tri-axial knit of 650 denier, 60 filament,no-twist Spectra 900 with 12 yarns per inch in weft direction, 12 yarnsper inch, each -45 and +45 degrees to the warp (no strength yarn in themachine direction) and 70 denier polyester knit around (not through) theSpectra strength yarns to facilitate handling of the web.

The resin was allowed to form in situ as in Example 1. The product was astrong, resilient composite demonstrating high puncture resistance.

The resin-forming composition in this example comprised, by volume, 50parts of diphenylmethane diisocyanate and 50 parts of polyol, primarilypolypropylene glycol.

EXAMPLE 3

A non-woven web of randomly oriented Spectra 1000 fibers approximately 2inches long and weighing approximately 5 ounces per square yard wassaturated with an aliphatic polyurethane casting compound, essentiallyas used in Example 1. Care was taken to avoid entrapment of visible airbubbles. The casting compound could also be introduced around the fibersby an atomized spray or any other known method.

The resulting composite material weighed 11 ounces per square yard,exhibited excellent penetration resistance, color stability and highflexibility. The composite is suitable for applications requiringflexibility and high strength such as the midsole of a shoe, or thelike.

EXAMPLE 4

A tri-axial knit fabric (+45 deg., 0 deg., -45 deg.) of 650 denierSpectra 900 weighing approximately 8 ounces per square yard was used asthe reinforced fiber for the composite. The fabric was knit so as toavoid crimp of the fibers, which increases load transmission in theresulting composite. An aromatic polyurethane compound essentially as inExample 2 was sprayed onto the fabric so as to penetrate the bundles ofthe yarns.

The resulting composite weighing approximately 14 ounces per square yardwas flexible and exhibited excellent penetration, abrasion, and cutresistance and tensile strength greater than either the matrix resin orreinforcing fabric alone. The composite so formed would be suitable forapplications in soft luggage, or the like.

It is to be noted that Spectra 900 and Spectra 1000 fibers were used inthe above examples. It is preferred to use Spectra 900 because suchfibers have a somewhat larger diameter (38 microns compared to 27microns for the Spectra 1000) and evidences less cutting of the resinmatrix on flexing and stressing of the composite. Spectra 1000 isactually stronger than the Spectra 900 but the canals through the voidfraction of a 900 fabric are larger in section perpendicular to theiraxes than through the void fraction of a 1000 fabric of equivalentstrength. The larger canals in the 900 fabric appear to facilitate thepositioning of the resin components to form the resin in situ and thusencapsulate the fibers.

The composites demonstrated in the above examples, broadly speaking,comprise a resilient, flexible or elastic polyurethane matrix formed insitu about a layer of high strength extended chain polyethylene fibers.The matrix and fibers are not bonded to each other and, because of thisand the high degree of lubricity demonstrated by the fibers, thecomposite has essentially the same degree of resilience or elasticity ofthe resin itself up to the point where the composite is bentsufficiently for the matrix to start bearing and braking on and aroundthe fibers (snubbing). At that point, any further bending of thecomposite spreads the load along to adjacent fibers or fiber segments.Then, when the bending stress is released, the energy accumulated in thefibers and their matrix is also released and the composite restoresitself to its original shape. The springy nature of Spectra fiberassists in this regard.

Flexibility of the composite results in large measure from the fact thatthere is little or no bonding between the fibers and matrix and theflexible or resilient nature of both materials. As the material isflexed, the fibers, on a microscopic scale, are free to bend and do notfunction to strengthen the matrix until the degree of bending is suchthat the fibers are squeezed or stubbed by the matrix. Up to this point,the fibers can flex and bend freely because they are essentiallyindependent of the matrix and flexibility is, at this stage, determinedpreponderantly by the characteristics of the resin matrix itself.However, with further bending, the fibers themselves begin to exert abraking action as they are squeezed by the bent matrix, thus serving toreinforce the bent composite and assist it to return to its originalform when the stress is relieved.

As noted, the fibers provide the composite with outstanding penetrationresistance. In this regard, the matrix functions to hold the fiberswithin the matrix so that any load applied to the composite, e.g. theload resulting from the application of a piercing object such as a nailor ice pick, is shared by the multiple fibers as a group. The appliedload is thus distributed over the multiplicity of fibers in accord withthe stress applied. This gives the composite a high degree of punctureresistance.

The composites of the invention are suitable for a broad array ofindustrial and consumer products, particularly those requiring punctureresistance and strength with flexibility. This includes such diverseproducts as luggage, footwear, body protection, boats, portablebuildings and greenhouses, highway delineators, buoys and the like. Thecomposites have exceptionally high strength:weight ratios and can beessentially abrasion- and puncture-proof. While the products asdescribed are resilient, i.e. sufficiently flexible to recover theiroriginal shape after deformation or flexing, they may also be made intorigid forms if such construction is desired.

A particularly useful application of the present composites is asreinforcement in the corners or other parts of luggage. Luggage isfrequently pierced in transport. The use of the present composites withtheir significant puncture resistance can effectively deal with thisproblem.

Another particularly significant area of use which depends largely onthe improved puncture resistance of the composites is in the provisionof protective clothing for law enforcement or the like where pointedobjects may be encountered. Similar advantage is contemplated in thecase of soles for shoes where puncture resistance against nails or thelike is important.

While the invention has been described above using a polyurethane matrixand extended chain polyethylene fibers, it will be appreciated thatvarious modifications may be made without departing from the scope ofthe invention. For example, other types of high strength fibers may beused to replace part or all of the polyethylene fibers, provided theseother fibers are such that they do not bond well to the matrix. Suchfibers may include, for example, textile fibers such as polyester, ormetal or ceramic fibers which may be pretreated so as to be non-adhesiveto the matrix. Advantageously, polyester fibers are used if a relativelythick composite is to be prepared (e.g. one which is 10 millimetersthick). It has been found that with such thicker composites, it isdifficult to control the exotherm, when a polyurethane matrix is formedin situ, to keep below the softening point of polyethylene fibers.

Additionally, while "Spectra" fibers have been exemplified, it should berecognized that other equivalent high strength polyethylene fibers, e.g.those available as "Dynema", may be used.

A filler material may also be included in the composite. This may be,for example, another fiber, whether random or oriented or both; apigment to provide color; glass microspheres or metal powder to reduceor increase density; resin plasticizers or modifiers; thixotropicagents; abrasives, etc., selected to provide the desired overallproperties. Thus, as an illustration, an in situ reinforced syntaticfoam may be made comprising a polyurethane matrix, extended chainpolyethylene fibers and plastic or glass microspheres.

It will be appreciated that, regardless of the modifications which maybe made, the composites of the invention should maintain certainessential characteristics. Firstly, there should be essentially noadhesion between at least the reinforcing portion of the fibrouscomponent and the matrix. This is an important distinction over priorcomposites where it has been considered that bonding was essential. Inthe present case, freedom from any substantive amount of bonding betweenthe reinforcing fibrous component and the matrix is essential so as toallow for interfacial movement or slippage between the resin and thefibrous component. This slippage is two directional, thus permittingrecovery of the composite after bending.

Another essential feature is that the resin matrix should envelop most,if not all, the reinforcing fibers. This envelopment may be aroundindividual fibers, typically the situation where random staple fibersare used. In this case, the side of each staple fiber slips back andforth at the interface with the enveloping resin. However, the matrixalso envelops the ends of the staple so that it cannot move to adifferent location within the matrix.

Where bundles of fibers in continuous filament form are used, typicallyin an oriented fashion where the filaments are held in position byanother yarn, e.g. in fabric form, the matrix will envelop each bundlebut not necessarily each fiber of the bundle. An exception to completeenvelopment may occur if it is desired to have the reinforcing fiberaccessible to the composite surface, for example, to take advantage ofthe lubricious character of extended chain polyethylene fibers.

The desired combination of high strength and flexibility in the presentcomposites apparently results from the fact that the fibers are notbonded to the matrix with consequent frictional effects between the twocomponents. Desirably, the matrix also has a much smaller Young'smodulus than the fibers. In this connection, it appears that the desiredcombination of strength and flexibility can be realized by multiplyingthe Young's modulus of the matrix material by 2/3 of the ratio of thelength to radius of the fiber, i.e. ##EQU1## where Y is Young's modulusfor the matrix, and 1 and r represent the length and radius of thefiber. If X is near or above the Young's modulus of the fiber (17×10⁶psi for Spectra 900 and 25×10⁶ psi for Spectra 1000), the strength ofthe matrix will approach that of the fiber times the percent volumeloading. The resulting material remains flexible while maintainingoptimum strength.

The scope of the invention is defined in the following claims wherein:

What is claimed is:
 1. A flexible composite comprising a high tensilestrength polyolefin or polyester fibrous component dispersed within aflexible polyurethane matrix, the fibrous component being incompletelyenveloped by said matrix so that at least some of said fibrous componentis accessible to the surface of the composite, the polyurethane matrixand fibrous component being essentially unbonded to each other so thatthe composite retains essentially the flexibility of the polyurethanematrix.
 2. The composite of claim 1 wherein the matrix comprisespolyurethane formed in situ about the fibrous component.
 3. Thecomposite of claim 1 comprising lubricous polyolefin fibers as thefibrous component incompletely enveloped by said polymer matrix, saidfibers being accessible at the surface of said composite to contributewear resistance to said surface.